D-Link DGS-3700 VLANs

DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual

VLANs

Priority tagging is a function defined by the IEEE 802.1p standard designed to pr ov ide a m eans of managin g traf fic on

a network where many different types of data may be transmitted simultane ously. It is intend ed to alleviate problem s

associated with the delivery of time critical data over congested networks. The quality of applicat ions that are

dependent on such time critical data, such as video conferencing, can be severely and ad versely affected by even

very small delays in transmission.

Network devices that are in compliance with the IEEE 802.1p s tandard have the ability to recognize the priority level of

data packets. These devices can also assign a priority label or tag to packets. Com pliant devices can also strip priority

tags from packets. This priority tag determines the packet's degree of expeditiousnes s and determines the queue to

which it will be assigned.

Priority tags are given values from 0 to 7 with 0 being assigned to the lowest priority data and 7 ass igned to the

highest. The highest priority tag 7 is generally only used for data associated with video or audio appl ications, which

are sensitive to even slight delays, or for data from specif ied end users whose data transm issions warrant special

consideration.

The Switch allows you to further tailor how priority tagged data p ac k et s ar e ha nd le d o n your network. Using queues to

manage priority tagged data allows you to specify its relative priority to suit the n eeds of your net work. There m ay be

circumstances where it would be advantageous to group two or more differently tagged pac kets into the sam e queue.

Generally, however, it is recommended that the highest priority queue, Queue 7, be res erved for data pack ets with a

priority value of 7. Packets that have not been given any priority value are placed i n Queue 0 and thus given the

lowest priority for delivery.

Strict mode and weighted round robin system are employed on t he Switch to determ ine the rate at which the queu es

are emptied of packets. The ratio used for clearing the queues is 4:1. This m eans that the highest priority queue,

Queue 7, will clear 4 packets for every 1 packet cleared from Queue 0.

Remember, the priority queue settings on the Switch are for al l ports, and all devices connected to t he Switch will be

affected. This priority queuing system will be especially beneficial if your network em ploys switches with t he capab ility

of assigning priority tags.

A Virtual Local Area Network (VLAN) is a network topology configur ed according to a logica l scheme rather than the

physical layout. VLANs can be used to combine any collection of LAN segm ents into an aut onomous user g roup that

appears as a single LAN. VLANs also logically segment the network into different broa dcast domains s o that packet s

are forwarded only between ports within the VLAN. Typically, a VLAN corr esp ond s to a particular subnet, although not

necessarily.

VLANs can enhance performance by conserving bandwidth, and im prove security by limiting traffic to specific

domains.

A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes that fr equently

communicate with each other are assigned to the same VLAN, regardless of where th ey are physically on the

network. Logically, a VLAN can be equated to a broadcast domain, bec ause broa dcast pack ets are f orwarde d to onl y

members of the VLAN on which the broadcast was initiated.

No matter what basis is used to uniquely identify end nodes and ass ign these nodes VLAN membership, pack ets

cannot cross VLANs without a network device performing a routing function between the VLANs.

The Switch supports IEEE 802.1Q VLANs and Port-Based VLANs. The port untagging f unction can be used to

remove the 802.1Q tag from packet headers to maintain compatibilit y with devices that are tag-unaware.

The Switch's default is to assign all ports to a single 802.1Q VL AN named "def ault." The " default" VLAN has a VID =

1. The member ports of Port-based VLANs may overlap, if desired.